1. Field of The Invention
The present invention relates to a method for producing methyl methacrylate. More particularly, the present invention is concerned with a method for producing methyl methacrylate, which comprises subjecting at least one starting material selected from the group consisting of isobutylene and tert-butanol to a gas phase catalytic oxidation reaction with molecular oxygen in the presence of a catalyst to thereby obtain gas (a) containing methacrolein gas and steam; introducing the gas (a) and a liquid mixture (I) containing liquid methacrolein and liquid methanol into a dehydration tower at a lower portion thereof and at an upper portion thereof, respectively, thereby allowing the liquid mixture (I) to flow downwardly in the dehydration tower and contact countercurrently with the gas (a) in the dehydration tower, to thereby obtain a dehydrated gas mixture (b) containing methacrolein gas and methanol gas; introducing the dehydrated gas mixture (b) and a liquid mixture (II) containing liquid methacrolein and liquid methanol into an absorption tower at a lower portion thereof and at an upper portion thereof, respectively, thereby allowing the liquid mixture (II) to flow downwardly in the absorption tower and contact countercurrently with the dehydrated gas mixture (b) in the absorption tower, so that substantially all of the methacrolein gas and methanol gas which are contained in the dehydrated gas mixture (b) is absorbed by the liquid mixture (II), thereby obtaining a liquid mixture (III) containing liquid methacrolein and liquid methanol; and subjecting the methacrolein and methanol, which are contained in the liquid mixture (III), to an oxidative esterification reaction in the presence of molecular oxygen and in the presence of a palladium catalyst.
By the method of the present invention, it has become possible to provide a methacrolein/methanol liquid mixture for an oxidative esterification reaction, which has a high methacrolein content, as compared to the methacrolein content of the methacrolein/methanol liquid mixtures provided by the conventional methods. Therefore, by the method of the present invention, methyl methacrylate can be produced with high efficiency. Further, in the method of the present invention, each of the above-mentioned liquid mixtures (I) and (II) introduced into the dehydration tower and the absorption tower, respectively, may individually be a liquid mixture (IV) containing liquid methacrolein and liquid methanol, which is obtained by separation from the reaction mixture produced by the above-mentioned oxidative esterification reaction. Therefore, in the method of the present invention, when the production of methyl methacrylate is continuously performed by recycling the liquid mixture (IV) (which is separated from the reaction mixture obtained by the oxidative esterification reaction) as the liquid mixtures (I) and/or (II), the amount of methanol needed for the continuous production process can be considerably reduced, as compared to that in the conventional methods, and the continuous production process can be performed without using complicated apparatuses for the separation and recovery of methanol, which are necessarily used in the conventional methods. This is advantageous not only in that the cost of producing methyl methacrylate can be considerably reduced, but also in that the production process is free from the troubles caused by the use of the complicated apparatuses for the separation and recovery of methanol, so that the desired methyl methacrylate can be produced stably.
2. Prior Art
For producing methyl methacrylate, which has a high commercial value, a so-called "via methacrylic acid process" has already been practiced on a commercial scale. The "via methacrylic acid process" comprises subjecting at least one starting material selected from the group consisting of isobutylene and tert-butanol to a gas phase catalytic oxidation reaction in the presence of molecular oxygen and in the presence of a catalyst to thereby obtain methacrolein; subjecting the obtained methacrolein to a gas phase catalytic oxidation reaction in the presence of molecular oxygen and in the presence of a catalyst to thereby obtain methacrylic acid; and reacting the obtained methacrylic acid with methanol to obtain methyl methacrylate.
On the other hand, recently, extensive and intensive studies have been made on a newly developed method for producing methyl methacrylate, which comprises subjecting methacrolein (ML) and methanol (MeOH) to an oxidative esterification reaction in the presence of molecular oxygen and in the presence of a catalyst, to thereby produce methyl methacrylate (MMA) by one step directly from methacrolein (ML) (this process is hereafter frequently referred to simply as a "direct ML-to-MMA process" and the above-mentioned oxidative esterification reaction is hereafter frequently referred to simply as a "direct ML-to-MMA synthesis reaction"?).
In this process (i.e., the direct ML-to-MMA process), a liquid mixture of liquid methacrolein and liquid methanol is introduced into a reactor, and a reaction is performed in the presence of molecular oxygen and in the presence of a palladium catalyst. However, the activity of the palladium catalyst used in this process is likely to be inhibited by water contained in the reaction system. Therefore, for increasing the production of methyl methacrylate, it is necessary not only to increase the methacrolein content of the above-mentioned liquid mixture, but also to lower the water content of the above-mentioned liquid mixture to a level as low as possible.
However, at present, as mentioned above, methacrolein is produced by subjecting at least one starting material selected from the group consisting of isobutylene and tert-butanol to a gas phase catalytic oxidation reaction in the presence of molecular oxygen and in the presence of a catalyst (this reaction is hereinafter frequently referred to simply as a "methacrolein synthesis reaction"). The methacrolein product obtained by the above-mentioned reaction contains a considerable amount of water in addition to by-products. The water contained in the methacrolein product is derived from various sources, such as water generated in the above-mentioned methacrolein synthesis reaction; steam used as a diluent gas; water generated in a dehydration reaction of tert-butanol (when tert-butanol is used as a starting material); and water contained in tert-butanol, which water is derived from an azeotropic tert-butanol/water mixture formed by a specific production process of tert-butanol. Thus, water is inevitably contained in the methacrolein product. Unless the water content of the methacrolein product is lowered by dehydration, it becomes impossible to produce methyl methacrylate efficiently by the direct ML-to-MMA process.
As mentioned above, when the production of methacrolein is conducted by the above-mentioned gas phase catalytic oxidation reaction, methacrolein is generally obtained in the form of gas containing methacrolein and steam. In order to obtain a methacrolein product having a low water content by the gas phase catalytic oxidation reaction, the water content of the above-mentioned gas is lowered, and gaseous methacrolein contained in the gas is separated and recovered in the form of a liquid mixture containing liquid methacrolein. However, in a commercial scale production of methacrolein, a treatment using a known desiccant, such as silica-alumina, zeolite or calcium chloride, cannot be employed as method for lowering the-water content of the gas containing methacrolein and steam, because such a known desiccant has a poor dehydrating ability, and an unfavorable polymerization of methacrolein is likely to occur on the surface of the desiccant.
For obtaining the methacrolein product having a low water content by way of lowering the water content of the methacrolein product during the production process for methacrolein, various proposals have been made. For example, Unexamined Japanese Patent Application Laid-Open Specification (Japanese Kokai) No. 49-92007 discloses a method in which gaseous methacrolein in the gas containing gaseous methacrolein and steam is absorbed into an alcohol and the resultant liquid mixture of liquid methacrolein and a liquid alcohol is subjected to an extractive distillation using water as an extraction solvent, to thereby separate and recover methacrolein. However, in this method, since water is used as an extraction solvent, the methacrolein is separated and recovered inevitably in the form of an azeotropic mixture of methacrolein and water [azeotropic point: 63.6.degree. C.; methacrolein/water (weight ratio): 100/7.9 ]. Therefore, in this method, it is impossible to obtain a methacrolein product having a water content which is lower than that of the azeotropic mixture of methacrolein and water. Further, U.S. Pat. No. 2,514,966 discloses a method in which gaseous methacrolein in the gas containing gaseous methacrolein and steam is absorbed into water. However, in this method, for the same reason as mentioned above in connection with Unexamined Japanese Patent Application Laid-Open Specification No. 49-92007, it is impossible to obtain a methacrolein product having a water content which is lower than that of the azeotropic mixture of methacrolein and water.
Unexamined Japanese Patent Application Laid-Open Specification (Japanese Kokai) No. 55-19213 discloses a method in which an organic compound having a high boiling point, such as an alkylnaphthalene, is used as a solvent, and methacrolein contained in the gas containing methacrolein and steam is absorbed into the solvent to thereby separate and recover methacrolein. However, in this method, during the separation of methacrolein from the mixture of methacrolein and the solvent, the mixture is kept under heated conditions, so that a polymerization of methacrolein is likely to occur, thereby consuming a large amount of methacrolein monomers.
Unexamined Japanese Patent Application Laid-Open Specification (Japanese Kokai) No. 56-87530 (corresponding to U.S. Pat. No. 4,329,513) discloses a method for dehydrating a gas containing an unsaturated aldehyde and steam, which gas is obtained by subjecting at least one starting material selected from the group consisting of propylene, isobutylene and tert-butanol to a gas phase catalytic oxidation reaction. Specifically, the method disclosed in this patent document comprises introducing a gas containing an unsaturated aldehyde and steam, and liquid methanol into a dehydration tower at a lower portion thereof and at an upper portion thereof, respectively, wherein the liquid methanol is introduced in an amount such that substantially all of the methanol can be gasified, thereby allowing the liquid methanol to flow downwardly in the dehydration tower and contact countercurrently with the gas in the dehydration tower, so that the gas is dehydrated to form a dehydrated gas containing the unsaturated aldehyde; and introducing the unsaturated aldehyde-containing dehydrated gas and liquid methanol into an absorption tower, so that the unsaturated aldehyde which is contained in the dehydrated gas is absorbed into the liquid methanol; and recovering the unsaturated aldehyde in the form of a liquid mixture containing the liquid unsaturated aldehyde and the liquid methanol.
However, in this method disclosed-in Unexamined Japanese Patent Application Laid-Open Specification No. 56-87530, since methanol is added to each of the gas containing the unsaturated aldehyde (methacrolein) in the dehydration tower and the dehydrated gas containing the unsaturated aldehyde (methacrolein) in the absorption tower, the unsaturated aldehyde is caused to be diluted with methanol, so that the content of the unsaturated aldehyde (methacrolein) in the liquid mixture (containing the liquid unsaturated aldehyde and the liquid methanol) withdrawn from the outlet of the absorption tower is on a level as low as 18 % by weight or less. On the other hand, in the commercial scale production of methyl methacrylate by the direct ML-to-MMA process, as a starting material for the direct ML-to-MMA synthesis reaction, it is preferred to use a methacrolein/methanol liquid mixture having a methacrolein content of not less than 25 % by weight and having a methacrolein/methanol weight ratio of not less than 0.33. That is, the method disclosed in Unexamined Japanese Patent Application Laid-Open Specification No. 56-87530 has a problem in that, although the water content of the methacrolein/methanol liquid mixture can be lowered, it is impossible to increase the methacrolein content of the methacrolein/methanol liquid mixture to a level which is sufficient for improving the productivity of methyl methacrylate in the direct ML-to-MMA process.
With respect to the methacrolein/methanol liquid mixture obtained by the method disclosed in Unexamined Japanese Patent Application Laid-Open Specification No. 56-87530, even if it is attempted to increase the methacrolein content of the methacrolein/methanol liquid mixture by a method in which the methacrolein/methanol liquid mixture withdrawn from the absorption tower is introduced into a distillation tower to thereby concentrate the liquid mixture, it is impossible to obtain a methacrolein/methanol liquid mixture having a satisfactorily high methacrolein content. The reason therefor resides in that, by the above-mentioned method using a distillation tower, only an azeotropic mixture of methacrolein and methanol (azeotropic point: 58.0.degree. C.; methacrolein/methanol weight ratio: 72.2/27.7) is obtained, so that it is impossible to obtain a methacrolein/methanol liquid mixture having a methacrolein content higher than that of the above-mentioned azeotropic mixture. Further, this method is disadvantageous not only in that a distillation tower is additionally needed, but also in that there is a danger that a polymerization of methacrolein occurs in the distillation tower, so that the resultant polymerized product causes troubles in the production system for methyl methacrylate, leading to a difficulty in stable production of methyl methacrylate.
Further, in this Unexamined Japanese Patent Application Laid-Open Specification No. 56-87530, the synthesis of methyl methacrylate by the direct ML-to-MMA process is conducted using an excessive amount of methanol, which is far larger than that consumed in the direct ML-to-MMA synthesis reaction. The reason for the use of an excessive amount of methanol resides in that a considerable amount of methanol is needed in the two preceding steps for dehydrating a gas containing a gaseous methacrolein and steam in a dehydration tower and for absorbing the gaseous methacrolein in an absorption tower. Due to the use of the excessive amount of methanol, it is necessary that a large amount of unreacted methanol be separated from the reaction mixture (containing desired methyl methacrylate) obtained by the direct ML-to-MMA synthesis reaction and that the separated methanol be recycled. Such a separation operation and a recycling operation consume a disadvantageously large amount of energy. On the other hand, it is noted that, when the amount of methanol used in the direct ML-to-MMA process is small, both the conversion of methacrolein and the selectivity for methyl methacrylate in the direct ML-to-MMA process tend to lower.
With respect to the method for separating methanol from the reaction mixture obtained by the direct ML-to-MMA synthesis reaction, and recycling the separated methanol to the dehydration tower and the absorption tower, the following methods have been proposed.
First, as described in Unexamined Japanese Patent Application Laid-Open Specification (Japanese Kokai) No. 58-157740, there is known a method for separating a liquid mixture containing liquid methyl methacrylate and unreacted liquid methanol from a reaction mixture (containing methyl methacrylate, water, methacrolein and methanol) obtained by the direct ML-to-MMA synthesis reaction. Specifically, this method comprises introducing the reaction mixture obtained by the direct ML-to-MMA synthesis reaction into a distillation tower to thereby obtain a liquid mixture containing liquid methyl methacrylate and unreacted liquid methanol from the bottom of the tower, and a liquid mixture containing unreacted liquid methacrolein and unreacted liquid methanol from the top of or an upper portion of the tower. In this method, the methacrolein/methanol liquid mixture obtained from the top of or an upper portion of the tower is recycled to a reactor for the direct ML-to-MMA synthesis reaction.
On the other hand, there is also a known method for separating methanol from the above-mentioned liquid mixture (containing liquid methyl methacrylate and unreacted liquid methanol) obtained by the separation from the reaction mixture produced by the direct ML-to-MMA synthesis reaction. For example, Unexamined Japanese Patent Application Laid-Open Specification (Japanese Kokai) Nos. 57-9739 (corresponding to U.S. Pat. No. 4,518,462) and 57-9740 disclose the following method. The liquid mixture containing liquid methyl methacrylate and unreacted liquid methanol is introduced into a first distillation tower, together with a C.sub.6 -C.sub.8 saturated hydrocarbon, to thereby distill substantially all of the methanol in the form of an azeotropic mixture thereof with the C.sub.6 -C.sub.8 saturated hydrocarbon from the top of the tower. The obtained azeotropic mixture is introduced into a phase separation vessel to thereby separate the mixture into an upper layer composed mainly of C.sub.6 -C.sub.8 saturated hydrocarbon and a lower layer composed mainly of methanol. The obtained C.sub.6 -C.sub.8 saturated hydrocarbon of the upper layer is recycled to the first distillation tower, and the obtained methanol of the lower layer is subjected to a distillation in a second distillation tower, wherein methanol is withdrawn from the bottom of the second distillation tower and recycled to the dehydration tower and the absorption tower. From the top of the second distillation tower, the C.sub.6 -C.sub.8 saturated hydrocarbon is distilled in the form of an azeotropic mixture thereof with methanol, which is recycled to the phase separation vessel.
However, when this method of Unexamined Japanese Patent Application Laid-Open Specification Nos. 57-9739 and 57-9740 is combined with the method of Unexamined Japanese Patent Application Laid-Open Specification No. 58-157740 so as to conduct a continuous production of methyl methacrylate, the production process becomes disadvantageously complicated and extensive, because this process needs not only a phase separation operation and distillation operations using three distillation towers, but also includes a plurality of recycling steps.
Further, the above method is disadvantageous in that, when the liquid mixture containing a C.sub.6 -C.sub.8 saturated hydrocarbon, liquid methyl methacrylate and liquid methanol is introduced into the first distillation tower, it is necessary to control the amount of the saturated hydrocarbon present in the tower, the amount of the liquid mixture to be introduced, the operation temperature of the distillation tower, the thermal input for heating and the amounts of products withdrawn from the top and bottom of the tower so that substantially all of the introduced saturated hydrocarbon is maintained at a position higher than a plate at which the liquid mixture is introduced into the tower. When a part or all of the saturated hydrocarbon is present at a position lower than a plate at which the liquid mixture is introduced into the tower, the water contained in the liquid mixture introduced into the first distillation tower disadvantageously undergoes azeotropy with the saturated hydrocarbon and, hence, the water enters the separated and recovered methanol, so that the water content of the separated and recovered methanol exceeds 3% by weight. When the methanol having such a high water content is introduced into the dehydration tower and the absorption tower, the water content of a liquid mixture (containing liquid methacrolein and liquid methanol) obtained from the absorption tower is increased, rendering the liquid mixture unsuitable for use as a feedstock for the direct ML-to-MMA synthesis reaction.
In addition, the above method has also the following disadvantage. When the operation conditions of the first distillation tower are changed or when the operation of the first distillation tower is started or stopped, the operation conditions of the tower become unsteady, thus causing the above-mentioned saturated hydrocarbon in the introduced liquid mixture to flow down inside the tower toward the bottom thereof through a plurality of plate regions of the tower. Since the saturated hydrocarbon is a poor solvent for the produced polymers and the by-produced polymers present in the introduced liquid mixture, a deposition of these polymers occurs, leading to a clogging of the plates and pipes of the first distillation tower. Frequently, this clogging is so serious as to make it impossible to continue the operation of the first distillation tower.
Moreover, the above method has a further problem in that a plurality of azeotropic systems having azeotropic points close to each other are disadvantageously formed in a distillation tower which is used for separating methanol from a reaction mixture (containing methyl methacrylate, water, methacrolein and methanol) obtained by the direct ML-to-MMA synthesis reaction, wherein the resultant separated methanol is intended to be used for dehydrating a gas containing methacrolein gas and steam and for absorbing methacrolein gas. Due to the formation of such plurality of azeotropic systems, it becomes difficult to stably operate such distillation tower. In one example, a plurality of azeotropic systems are formed in the distillation tower used for separating and obtaining a liquid mixture containing unreacted liquid methacrolein and unreacted liquid methanol from the above-mentioned reaction mixture obtained by the direct ML-to-MMA synthesis reaction, which distillation tower is described in Unexamined Japanese Patent Application Laid-Open Specification (Japanese Kokai) No. 58-157740. In another example, a plurality of azeotropic systems are also formed in the first and second distillation towers used in the method of Unexamined Japanese Patent Application Laid-Open Specification (Japanese Kokai) Nos. 57-9739 and 57-9740. More specifically, the below-mentioned azeotropic systems are formed in the above-mentioned distillation tower used (for separating and obtaining a liquid mixture containing unreacted liquid methacrolein and unreacted liquid methanol) in the method of Unexamined Japanese Patent Application Laid-Open Specification (Japanese Kokai) No. 58-157740 (the temperatures in parentheses show azeotropic points).
methanol/acetone (55.5.degree. C.), PA1 mathacrolein/methanol (58.0.degree. C.), PA1 water/methacrolein (63.6.degree. C.), PA1 methyl methacrylate/methanol (64.5.degree. C.), PA1 methyl methacrylate/water (83.0.degree. C.), and PA1 methacrylic acid/water (99.3.degree. C.). PA1 n-hexane/methanol (49.9.degree. C.), PA1 water/n-hexane (61.6.degree. C.), PA1 methyl methacrylate/methanol (64.5.degree. C.), PA1 methyl methacrylate/water (83.0.degree. C.), and PA1 methacrylic acid/water (99.3.degree. C.). PA1 n-hexane/methanol (49.9.degree. C.), and PA1 water/n-hexane (61.6.degree. C.). PA1 mathacrolein/n-hexane (56.1.degree. C.), PA1 water/isobutylaldehyde (64.3.degree. C.), PA1 methanol/isobutylaldehyde (62.7.degree. C.), PA1 water/methacrolein (63.6.degree. C.), PA1 methyl isobutyrate/methanol (64.0.degree. C.), and PA1 water/methyl isobutyrate (77.7.degree. C.).
Further, when n-hexane is used as the saturated hydrocarbon in the method of Unexamined Japanese Patent Application Laid-Open Specification (Japanese Kokai) Nos. 57-9739 and 57-9740, the below-mentioned azeotropic systems are formed between the main components of the liquid mixture in the first distillation tower (the temperatures in parentheses show azeotropic points).
In addition, in the method of Unexamined Japanese Patent Application Laid-Open Specification (Japanese Kokai) Nos. 57-9739 and 57-9740, the below-mentioned azeotropic systems are formed between the main components of the above-mentioned upper methanol layer in the second distillation tower (the temperatures in parentheses show azeotropic points).
Also, it is possible that the below-mentioned azeotropic systems are formed between trace substances (such as by-products) in each of the above-mentioned distillation towers (the temperatures in parentheses show azeotropic points).
Therefore, due to the formation of these azeotropic systems, when the conditions in the distillation tower are caused to change, e.g. when the internal temperature of the distillation tower changes by several centigrades, the composition of the distillate changes, so that it becomes difficult to stably operate the distillation tower. Further, trace substances, such as by-products, enter and are accumulated in the recycled system by azeotropy between the main components of the distillate, so that the composition of the distillate is caused to change, leading to an increased difficulty in stable operation of the distillation tower. For example, in the method of Unexamined Japanese Patent Application Laid-Open Specification (Japanese Kokai) Nos. 57-9739 and 57-9740, when a trace amount of methacrolein in the first distillation tower undergoes azeotropy with n-hexane and the resultant azeotropic mixture enters the subsequent separation vessel, problems arise not only in that the compositions of the upper layer liquid and the lower layer liquid obtained in the separation vessel are likely to change, but also in that the methacrolein accumulates in the separation vessel, so that the change in the compositions of the upper layer liquid and the lower layer liquid is amplified with the lapse of the operation time, rendering it very difficult to stably operate the distillation tower.
As is apparent from the above, in the conventional methods in which separated methanol is used for dehydrating a gas containing methacrolein gas and steam and for absorbing methacrolein gas in order to obtain a liquid mixture (containing liquid methacrolein and liquid methanol) which has a high methacrolein content and a low water content and hence is suitable for use as a feedstock for the direct ML-to-MMA synthesis reaction, problems arise not only in that a large amount of methanol is necessary, but also in that the production process for methyl methacrylate becomes complicated and extensive, and the stable operation of a production plant for methyl methacrylate becomes difficult. Thus, it has conventionally been difficult to commercially carry out the direct ML-to-MMA process.